Confusion over closure

[Tim Hawes]

So, I have been re-brushing my common lisp, and have been wanting to write some AWS api code in the process. Common Lisp has the aws-sign4 package that can be used to sign requests for their API.

The problem I am having is with a closure. aws-sign4 example code fails with the error message "Please bind *aws-credentials* to a function." It is initiated as nil, and the macro in the example is supposedly re-assigining the variable as a function, but it fails (and checking the output, *aws-credentials* is still nil).
https://github.com/copyleft/aws-sign4/blob/master/example/example.lisp

This code, is what I wrote on my own, and is identical (I think) to what aws-sign4 is suppose to be doing. This code works, and the global *foo* variable gets assigned to a function (within scope) using a macro: http://pastebin.com/ipzQHfj8

any insight as to what I am missing, and what helpful comments I might send to the maintainer of aws-sign4 to fix their example?

[Kaz Kylheku]

On 2016-12-01, Tim Hawes <trh...@gmail.com> wrote:
> So, I have been re-brushing my common lisp, and have been wanting to
> write some AWS api code in the process. Common Lisp has the aws-sign4
> package that can be used to sign requests for their API.
>
> The problem I am having is with a closure. aws-sign4 example code
> fails with the error message "Please bind *aws-credentials* to a
> function." It is initiated as nil, and the macro in the example is
> supposedly re-assigining the variable as a function, but it fails (and
> checking the output, *aws-credentials* is still nil).
> https://github.com/copyleft/aws-sign4/blob/master/example/example.lisp

Are you using that example exactly, with example/package.lisp
being loaded, and the (in-package :aws-sign4-example) top and all?

I'm suspecting that maybe somehow your *aws-credentials* symbol is not
aws-sign4:*aws-credentials*, but some-other-package:*aws-credentials*.
Thus you're not actually binding the correct symbol.

The source code text which defines your with-aws-credentials macro has
to be read under a package in which the symbol
aws-sign4:*aws-credentials* is visible. This means that whatever
package is current at that point either has to be using aws-sign4
(without shadowing that symbol) or else be importing that symbol
individually from aws-sign4.

In the verbatim example, this is arranged because:

- the current package is aws-sign4-example, and that package
uses aws-sign4 (via the :use clause in its defpackage); and
- the aws-sign4 exports the *aws-credentials* symbol.

You can always have your macro refer to the fully qualified symbol,
like this:

(defmacro with-aws-credentials (&body body)
`(let ((aws-sign4:*aws-credentials* (credentials-from-file)))
,@body))

If this change makes the "Please bind ..." go away, it's a
package-related problem.

[Tim Hawes]

Bingo! Thank you! I was not using the asdf system to test the example, so a definite namespace mismatch. Changing it aws-sign4:*aws-credentials* worked. Thanks again!